An Electro-Static Discharge (ESD) protection circuit is an important part of TFT (thin film transistor)-Liquid Crystal Display (TFT-LCD) panel and emerging Active Matrix/Organic Light Emitting Diode (AMOLED) panel, and protects display devices from electrostatic damage in manufacturing, transporting and operating.
FIG. 1 is a diagram of an ESD protection circuit in a usual display panel with only a Vcom line, and FIG. 2 is a diagram of a display panel with a VGH line and a VGL line, and the display panel usually comprises a gate driving circuit. The ESD protection circuit in the display panel shown in FIG. 1 operates as follows: in normal operation of the display panel, only leakage current or small positive current is discharged from the Vdata line (or Vgate line) to the Vcom line. When an ESD occurs, positive charges and negative charges are discharged quickly to the Vc line respectively so as to protect interior pixel units in array from damage. The ESD protection circuit in the display panel shown in FIG. 1 is simplest in structure, but has a major disadvantage, that is, when there is a voltage difference larger than threshold voltage Vth of a TFT between the Vdata line (or Vgate line) and the Vcom line, there may be a forward current discharging to the Vcom line, which has adverse effect on the display panel itself and the peripheral driving circuit thereof. The difference of the ESD protection circuit in the display panel shown in FIG. 2 from the ESD protection circuit in the display panel shown in FIG. 1 is that, when an ESD occurs, positive charges and negative charges discharge quickly to the VGH line and the VGL line respectively, but in normal operation of the display panel, there is small leakage current flowing from the Vdata line (or Vgate line) to the VGH line and the VGL line.
U.S. Pat. No. 6,515,644 discloses several conventional ESD protection circuits, as shown in FIGS. 3 and 4.
The ESD protection circuit shown in FIG. 3 is formed by a TFT B101 and a TFT B102 each of which is connected to form a diode, and operates as follows: when there are positive charges accumulated on the Vdata line (or Vgate line), the TFT B102 is turned on, the positive charges on the Vdata line (or Vgate line) are discharged to the Vcom line, and when there are negative charges accumulated on the Vdata line (or Vgate line), the TFT B101 is turned on, the negative charges on the Vdata line (or Vgate line) are discharged to the Vcom line. However, in normal operation of the display panel, above problem also exists, that is, when there is a voltage difference larger than threshold voltage Vth of a TFT between the Vdata line (or Vgate line) and the Vcom line, there may be a forward current discharging to the Vcom line, which has adverse effect on the display panel per se and the peripheral driving circuit thereof.
The ESD protection circuit shown in FIG. 4 resolves the leakage problem in normal operation of the display panel to some extent, and operates as follows: when there are positive charges accumulated on the Vdata line (or Vgate line), the TFT B202 applies a positive voltage to the gate of the TFT B203 so that the TFT B203 is turned on, thereby charges are discharged from the Vdata line (or Vgate line) to the Vcom line; and when there are negative charges accumulated on the Vdata line (or Vgate line), the TFT B201 applies a positive voltage to the gate of the TFT B203 so that the TFT B203 is turned on, thereby charges are discharged from the Vdata line (or Vgate line) to the Vcom line. Unlike the ESD protection circuit shown in FIG. 3, in the normal operation of the display panel shown in FIG. 4, only when there is a positive voltage difference larger than 2 times threshold voltage Vth of a TFT (i.e., 2 Vth) between the Vdata line (or Vgate line) and the Vcom line, a forward current may be discharged to the Vcom line, which resolves the leakage problem in normal operation state of the display panel to some extent.
Above ESD protection circuits largely eliminate the harm brought by ESD in a-Si (amorphous silicon) TFT LCD process. In a Low Temperature Poly-silicon (LTPS) TFT display panel integrated with a gate driving circuit, a VGH line and a VGL line are usually provided therein, thus the ESD protection circuit may be more properly designed.
For example, as shown in FIG. 5, an ESD protection circuit is formed by a TFT B301 and a TFT B302 each of which forms a two-terminals TFT diode element by connecting the gate and the drain thereof. For an enhanced TFT, in normal operation of the display panel display panel, no forward current is discharged to the Vcom line, and only extreme weak reverse leakage current is leaked to the VGH line and VGL line. When an ESD occurs, positive charges and negative charges are discharged rapidly to the VGH line and VGL line through the TFT B301 and the TFT B302 respectively.
However, as to the emerging oxide TFTs, serious leakage problem exists in all above ESD protection circuits wherein each of the TFTs forms a TFT diode, because current oxide TFTs usually are depletion-type devices, and for the depletion-type TFTs, when Vgs=0V, the TFTs are turned on. That is, each of the TFT diodes in above ESD protection circuits no longer can be regarded as a diode, because they are turned on regardless of whether the voltage between the two lines is positive or negative. For this reason, if existing ESD protection circuits are used for designing a display panel, in normal operation of the display panel, a large current will be discharged from the Vdata line (or Vgate line) to the Vcom line (or VGH line and VGL line), so that the pixel units in array inside the display panel may operate abnormally, and the periphery driving circuits of the display panel may be damaged.
At present, in the ESD protection circuits designed by using the depletion-type TFTs, the serious leakage problem is solved to some extent by forming a voltage dividing circuit. In the ESD protection circuit shown in FIG. 6, the drain of the TFT B401 is connected to the VGH line, and its advantage is that, the voltage dividing circuit will not absorb the current on the Vcom line in normal operation of the display panel; but its disadvantage is that, when there are positive charges to be discharged on the Vdata line, the maximum value of Vnet1 is limited by VGH, thus the degree of turning on for the TFT B403 is low, and charges are discharged slowly.